THE EFFECT OF STOCKING ON STOCKING WITH OTHER SPECIES Marian Leopold

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Marian Leopold. THE EFFECT OF EEL STOCKING ON STOCKING WITH OTHER FISH SPECIES. Vie et Milieu / Life & Environment, Observatoire Océanologique - Laboratoire Arago, 1986, pp.295-297. ￿hal-03024201￿

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. VIE MILIEU, 1986, 36 (4) : 295-297

THE EFFECT OF EEL STOCKING ON STOCKING WITH OTHER FISH SPECIES

Marian LEOPOLD Inland Fisheries Institute, blok 5 10-957 Olsztyn-Kortowo, Poland

ANGUILLA ANGUILLA RÉSUMÉ. — L'analyse de l'efficacité de l'alevinage en Anguille et en diverses

ALEVINAGE espèces de Poissons, pour 31 lacs d'une surface totale de 5 300 ha, a été effectuée à

EAUX CONTINENTALES partir d'échantillons obtenus durant plusieurs années. On a déterminé l'impact de l'alevinage intensif en Anguille sur la réussite de l'alevinage sur diverses espèces de Poissons. L'influence positive des Anguilles sur l'efficacité de l'alevinage des lacs avec Coregonus albula, C. lavaretus + C. peled, Stizostedion lucioperca et Tinca tinca, a été constatée. Par contre, aucune influence sur l'efficacité de l'alevinage de Cyprinus carpio et Carassius carassius n'a pu être décelée.

ANGUILLA ANGUILLA ABSTRACT. — Long-term data on fish stocking and commercial catches from 31

FISH STOCKING lakes of a total area of 5.300 ha were used to analyse the effect of stocking with eel

INLAND WATERS on other fish species. The effect of stocking at high rates with eel upon the effects of stocking with other fish species was determined. It was found that high rates of stocking with eel affected the effectiveness of stocking with vendace, whitefish + peled, pikeperch and tench positively, whilst no effect was found as regards stocking with and crucian carp.

INTRODUCTION and less and less like traditional fishery. Artificial stocking of many species has become a popular measure treated as a method of counteracting unfavourable changes taking place in the fish stocks Eel stocking into Polish lakes has been a common (and catches) in lakes under the effect of man-made practice since many years, carried out on a large influences, and especially under the effect of accele- scale. Area of lakes with no eel stocking in very rated . During the period 1952-1982 small. Average eel landings (commercial and re- only 4 lakes (of total area 50.4 ha) belonging to the creational) in lakes stocked with this species amount farm under study were not stocked with any fish to 7.0 kg/ha annually on a national scale, compared species. Ail other lakes were artificially stocked to less than 0.5 kg/ha in lakes with no eel stocking. although with varying rates. The farm used 13 fish Consequently, eel management is of considérable species and 1 crayfïsh species. économie significance in Poland and constitutes a subject of both theoretical and applied studies. In ail Many earlier studies (e.g. Leopold, 1975) revealed cases significant corrélation was found between eel significant positive corrélation between eel catches stocking and catches. and catches of other . This dependence was looked upon as resulting from lake trophy. Accor- This paper deals with one State Fish Farm which ding to the interprétations given so far, lakes of a manages 52 lakes of total area 5 828 ha. Lakes higher eutrophication were stocked with eel at a stocked with eel represent 59.6 % of the total lake higher rate, hence its fishery production was higher. number (31 lakes) and 91 % of the total lake area. At the same time, higher catches of other fish species At présent, lake fishery management is changing were considered as being caused by the higher its character and becoming more like aquaculture trophy of thèse lakes. 296 M. LEOPOLD

This view, although quite logical, implied that vertically (n represented the number of years, and there was no significant effect of increasing eel the variables — level of stocking and catches in population (due to increasing stocking) upon the particular years) and horizontally (n represented the lake biocenoses, and especially upon their ichthyo- number of lakes, and the variables — mean annual cenoses. However, eel catches at présent are much stocking and catch in particular lakes). Effectiveness higher than they used to be with natural recruite- of stocking with the given fish species was expressed ment; so density of eel stocks must have also in financial terms (cost of the stocking per 1 kg of increased. Possible effects of thèse dense stocks on the catch). In order to eleminate accidentai varia- the stocks of other fish species therefore cannot be tions, the time séries were smoothed using moving disregarded and necessitate proper studies. Spécial averages, so as to expose the trends of a more attention should certainly be devoted to the effect gênerai character. of intensive eel management on the management No attention was given to the fishing intensity. In with other fish species. the Fish Farm under study the managing team, the fishermen, and the management have not changed for many years, and cannot have any significant effect on the results obtained. SPECIES USED AND METHODS OF ANALYSIS

Analysis presented in this paper embrace stoc- RESULTS kings with 7 fish species i.e. : vendace Coregonus albula L., whitefish + peled C. lavaretus L. + C. peled Gmel., tench Tinca tinca L. ; pikeperch Stizoste- Overall results of the analysis are presented in dion lucioperca L., common carp Cyprinus carpio L. Table I. and crucian carp Carassius carassius L. Stockings with thèse species were analysed in lakes which were A. Lakes stocked with eel and vendace also stocked with eel. Analysis were performed taking advantage of the The dependence between stocking with vendace linear corrélation / régression /. In each case a and vendace catches one year later was highly sélection was made of ail lakes stocked with eel and significant so that, obviously, the measure was the given fish species , with no attention paid to effective. The dependence between eel stocking and other species. In this way, groups of lakes were catches was also very highly significant. Highly distinguished, for which studies were made on a significant négative corrélation was found between long-term scale of 31 years, each group being treated the effectiveness of stocking with vendace and rates as a whole. Successive steps of the analysis embra- of stocking with eel. Hence, the higher the rate of ced the following déterminations : (1) dependence stocking with eel, the lower the cost of vendace between rate of stocking with the given species and stocking needed to obtain 1 kg of vendace catch. level of its catches (2); dependence between rate of stocking with eel and level of eel catches (3), depen- B. Lakes stocked with eel, whitefish and peled dence between rate of stocking with eel and the effectiveness of stocking with the given fish species. Whitefish and peled were analysed together The latter dépendance was analysed in two ways, i.e. because the fishermen are frequently unable to

Table I. — Relationship between stocking with eel and eel catch as well as with stocking of other fish in 52 Polish lakes. The effect of stocking is expressed by the coefficient of corrélation (r). p = significance, f = degrees of freedom, n = number of lakes stoked with the given species and eel, in parenthesis — total number of lakes stocked with the given species, ha = area of lakes stocked with the given species and eel, in parenthesis — total area of lakes stocked with the given species, years = years of stocking/catch.

Effect of eel stocking Species Species stocking — species catch Eel stocking-eel catch on effectiveness stocking of species

r P f years n ha r*** P f n ha r*** p f

Coregonus albula 0.6710 0.001 28 52-81/53-82 11 3340.2 0.7600 0.001 25 11 3340.2 - 0.7519*<0.001 25 Coregonus lavaretus + C. pe- led 0.8641 0.001 25 53-79/56-82 22 (24) (4754.6) 0.7635 0.001 25 22 4706.9 - 0.5495* < 0.01 19 Slizostedion lucioperca 0.4458* 0.02 24 52-81/53-82 13(15) (1892.1) 0.6617 0.001 24 13 1844.5 - 0.5571* <0.01 24 Tinca tinca 0.8358* 0.001 22 55-82/55-82 26 (38) (5295.5) 0.7770 0.001 25 26 4862.2 - 0.6469*< 0.001 22 Carassius carassius 0.5976* 0.001 25 52-82/52-82 18(26) (2761.7) 0.7435 0.001 24 18 2396.7 no dependence Cyprinus carpio 0.7976** 0.001 23 53-81/54-82 13(18) (2111.2) 0.6305 0.001 24 13 1881.7 no dependence

* 5-year moving averages; ** 3-year moving averages; *** years : 52 or 53-78/56 or 57-82. EFFECT OF EEL STOCKING ON OTHER FISH SPECIES 297 distinguish between the two species. The dépen- led that in each of the 31 lakes stocked with eel, dance between whitefish + peled stockings and relatively higher rates of stocking with eel were catches was very highly significant, indicating a coupled with lower cost of stocking with other fish strict dependence of the catch on artificial stocking. species to obtain the same catch, i.e. with higher The rate of eel stocking and effectiveness of stocking effectiveness of stocking with thèse species. with whitefish and peled were significantly negati- vely correlated. Thus, stocking with eel had a bénéficiai effect on the effectiveness of stocking with the two coregonids, just as in case of the vendace. DISCUSSION

C. Lakes stocked with eel and tench In the 31 lakes under study, no négative effect of eel stocking on other fish species was noted. On the The dependence between tench stocking and contrary, it was found that with the exception of catches was very highly significant. Strict corrélation carp and crucian carp, stocking with eel improved was also found between eel stockings and catches. effects of stocking with other species. This statement Finally, a significant négative corrélation was found applies even to vendace, contradicting common between the rate of eel stocking and the cost of tench opinion that eel and vendace are unable to coexist. stocking. Hence, eel stocking influenced positively As regards carp and crucian carp, intensive stocking also the effectiveness of stocking with tench. with eel had no effect on the two species. The relationship between the rate of stocking with D. Lakes stocked with eel and pikeperch eel and the effectiveness of stocking with other fish species does not contradict earlier suggestions that Dependence between pikeperch stocking and this might be the resuit of a higher trophical catch was noticeable and a relation between eel condition of the lake. However, an opposite hypo- stocking and catches in the same lakes was also thesis is also possible. It can certainly by assumed significant. Stocking with eel correlates negatively that fishery production in lakes (in this case pro- with the effectiveness of stocking with pikeperch. duction of other fish species) increased due to the Consequently, it seems that eel has similarily a bénéficiai effect of eel stocking, or of the rather bénéficiai effect on the effects of stocking with intensive eel management upon the lake ecosystems pikeperch. and their ichthyocenoses. This effect is probably of complex and multiple character, as reflected by the E. Lakes stocked with eel and carp fact that it is noted for a variety of fish species differing as regards their requirements and rôle A strict dependence was found between carp played in the ecosystems, such as the coregonids, stocking and carp catches. No statistically significant tench and pikeperch. The conclusion is based on dependence was found between rate of eel stocking suggestions that eel can act as an ameliorator of the and effectiveness of stocking with carp, the latter fish stocks in lakes. In the course of eutrophication being expressed by various différent methods. This the predatory species gradually disappear from the fact suggests that the eel has no effect whatsoever lake environment. It is well known that thèse species on the effects of stocking the lakes with carp. are bénéficiai for the lake biocenosis and the maintenance of the fish stock balance. Moreover, lake eutrophication usually induces excessive deve- F. Lakes stocked with eel and crucian carp lopment of less valuable species, which are as a rule characterized by higher résistance : they develop at A strict dependence was found between crucian the expense of more valuable species, which are carp stocking and catches. As in the case of common biologically less résistant. Hence, it may be conclu- carp, no dependence was found between rate of ded that eel gradually takes over the part of preda- stocking with eel and the effectiveness of stocking tory species and becomes a regulator of the envi- with crucian carp. This fact allows us to state that ronmental balance and of the fishery production in the eel has no négative effect on the effectiveness of eutrophic lakes. stocking with crucian carp.

G. Horizontal approach

When the average effectiveness of stocking (over REFERENCES the 31-year period) with particular fish species was related to the respective average rate of stocking with eel, significant négative corrélation was found, LEOPOLD M., 1975. Ogôlny wplyw eksploatacji rybackiej the corrélation coefficient being r = — 0.4944 at na zrôznicowanie pilowôw wegorza w jeziorach pols- p < 0.001. Hence, a horizontal approach also revea- kich. Rocz. Nauk Roi, 96-H-4 : 47-56.